Polyesters are an important class of industrial chemicals. For example, polyalkylene terephthalates, derived from dicarboxylates and glycols, are widely used in thermoplastic fibers, films and molding applications because of their excellent physical properties and surface appearance. Polyethylene terephthalate (PET) is the most important commercial polyester within this class of polymers.
Polyethylene terephthalate is commonly prepared by one of two routes: (1) transesterification of dimethyl terephthalate (DMT) with ethylene glycol to form the intermediate bis-hydroxyethyl terephthalate, followed by polymerization (polycondensation) to form the polyethylene terephthalate; or (2) by direct esterification of terephthalic acid (TPA) with ethylene glycol, again followed by the polymerization step. Excess ethylene glycol is then typically recovered and recycled. Typically the transesterification (or esterification) and polymerization steps are carried out in the presence of one or more catalysts to produce a molten PET. The molten PET is then extruded and/or formed into the desired product. The polymerization and processing steps are carried out at a high temperature, for example, in the range of 200°–300° C. Under these conditions, some breakdown of the polymer occurs, leading to the formation of small amounts of aldehydic impurities such as acetaldehyde. These impurities may end up in the PET, or in the recovered ethylene glycol.
One of the major applications for PET resins is that of containers for potable beverages such as soft drinks, juices and water. Once having been formed in the PET polymer, any acetaldehyde in the container migrates over a period of time into the beverage. Even a trace amount of acetaldehyde gives the beverage an unpleasant aroma or taste, particularly noticeable in the case of mineral water. This problem has been minimized by the use of low molecular weight PET, reduction in exposure to heat during processing, and use of amine-based acetaldehyde scavengers, but these approaches have been only partially successful.
An additional problem caused by the presence of acetaldehyde in PET is color formation. This problem can be partly caused by acetaldehyde interaction with titanium compounds, frequently used as catalysts in producing PET.
For example, U.S. Pat. No. 6,113,997 discloses producing a melt-polymerized prepolymer having an intrinsic viscosity of 0.38–0.46 dl/g followed by solid-state polymerization to a polymer with an intrinsic viscosity of 0.60–0.90 dl/g and an acetaldehyde content equal to or less than 1 ppm.
WO 01/30900 discloses a process for reducing acetaldehyde content in a beverage in a polyester-based container by incorporating an oxidation catalyst active for the oxidation of acetaldehyde to acetic acid.
However, there is a constant need for new polyester substantially free of an aldehyde compound.